A rectifier transformer and a rectifier are generally used to form a rectifying circuit in the existing technology in order to convert AC power supply into DC power supply. FIG. 1 is the six-pulse wave bridge rectifier circuit diagram in the existing technology. As shown in FIG. 1, the rectifier transformer and the rectifier form a rectifier unit. Within an AC cycle or a 360° electric angle, the DC voltage Vdc contains six pulse ripples, and the electric angle interval of each ripple is 60°. During rectification, the rectifier will transmit a large quantity of harmonic currents to the power grid. In order to reduce the harmonic currents transmitted to the power grid, one of effective methods is to connect multiple rectifier units in parallel; wherein, the valve side voltage of each transformer shall be phase-shifted, and the harmonic currents between the connected rectifiers in parallel can be mutually counteracted on the power grid side. Two units form a 12-pulse rectification; four units form a 24-pulse rectification; and eight units form a 48-pulse rectification, etc.
FIG. 2 is the 12-pulse rectifier circuit composed diagram of two 6-pulse wave bridge rectifier units connected in parallel in the existing technology. As shown in FIG. 2, the phase angle difference between valve-side winding) L1 (0° voltage and valve-side winding) L2 (30° voltage is 30° in the 12-pulse rectification composed of two rectifier units connected in parallel. Within an AC cycle, the DC voltage Vdc contains 12 pulse ripples, and the electric angle interval of each ripple is 30°. When rectifier units are connected in parallel, the valve-side winding phase angle voltage difference (caused by different voltage phase angles) generates a ring current (sixfold power frequency) between parallel connection units. This sort of ring current caused by phase angle voltage difference will impact or interfere with normal working of rectifiers. The balance reactor I-T in FIG. 2 is one of effective methods for reducing the ring current caused by phase angle voltage difference.
Another sort of 12-pulse rectification mode is serial connection of two rectifier units. Compared with the parallel connection rectification mode, the serial connection rectification mode does not have the parallel connection ring current between rectifiers, but the resistance loss of serial connection rectifiers is increased by 100%, so the serial connection rectification mode is less used actually.
FIG. 3 is the schematic diagram of two sets of valve-side output windings that are axially split of one transformer (one phase, no iron core indicated) in the existing technology. As shown in FIG. 3, one transformer includes two sets of valve-side output windings; wherein, L1 and L2 are two sets of valve-side output windings that are radially (radius direction) split and H is the gird-side input winding, thus both the rectifier transformer cost and the floor area of the transformer are reduced. However, for the two sets of valve-side output windings of the same iron core, when L1 and L2 are connected in star shape and triangle shape respectively, the transformation ratio voltage difference (voltage deviation of star-shaped and triangular windings 1:√3) caused by taking the number of turns of star-shaped and triangular windings as an integer generates another sort of ring current (direct current) between parallel connection rectifiers. As shown in FIG. 3, L1 and L2 are radially (radius direction) split windings, the magnetic leakage impedance (the impedance restricting transformation ratio voltage difference from generating parallel connection ring current) between radially split valve-side windings is small, thus easily leading to a large transformation ratio voltage difference ring current. The transformation ratio voltage difference ring current will result in current imbalance (or inequality) between parallel connection rectifiers. On one hand, current imbalance will reduce the operational capability of parallel connection rectifying devices; on the other hand, the quintuple and septuple harmonic currents on the grid-side cannot be completely counteracted mutually and the uncounteracted quintuple and septuple harmonic currents are still transmitted to the power grid. Therefore, the transformation ratio voltage difference ring current must be reduced and controlled in the design and manufacture of parallel connection rectifier units.
FIG. 4 is the schematic diagram of two sets of valve-side output windings that are axially split of one transformer (one phase, and no iron core indicated) in the existing technology. As shown in FIG. 4, the structure of two sets of valve-side output windings of the transformer are axially split windings. L1 and L2 in FIG. 4 are two sets of valve-side axially (circle's center axis) split output windings, and H is parallel connection axially split input windings on the grid-side. The magnetic leakage impedance between axially split windings on the valve-side is large and can effectively restrict the ring current generated by transformation ratio voltage difference and phase angle voltage difference.
1500V DC power supply rectifier transformers for metro traction use axially dual split windings. Each rectifier transformer and two rectifiers form a 12-pulse rectification. There are totally two sets of 12-pulse rectifier units. The grid-side windings of the rectifier transformer include phase shift coils. With them, the phase angle of the valve-side voltage of the two sets of 12-pulse rectifying devices is phase-shifted by 15°. Two sets of 12-pulse rectifying devices are connected in parallel to form a 24-pulse rectification. FIG. 5 is 24-pulse rectification circuit diagram composed of two sets of 12-pulse rectifying devices connected in parallel in the existing technology. As shown in FIG. 5, within an AC cycle, the DC voltage Vdc contains 24 pulse ripples, and the electric angle interval of each ripple is 15°. Due to the magnetic leakage impedance between the axially split valve-side windings of the rectifier transformer is large, and the appropriate number of turns of star-shaped and triangular windings is selected, a balance reactor between parallel connection rectifiers may not be used. However, the ring current (or current inequality) between the rectifiers generated by the transformation ratio voltage difference of the star-shaped and triangular windings on the valve-side causes that the 24-pulse rectifier system still transmit the uncounteracted quintuple and septuple harmonic currents to the power grid.
At present, there is not a feasible technical scheme for realizing one rectifier transformer composed of the parallel connection 24-pulse rectification with four sets of valve-side windings and effectively controlling (or eliminating) ring currents and harmonic currents transmitted to the power grid. The main restricting factors are as follows: the magnetic leakage impedance of the radially split windings among the four sets of valve-side windings is small, and the transformation ratio voltage difference generated by taking the number of turns of star-shaped and triangular windings as an integer generates a large parallel connection ring current between rectifiers, so that rectifier units are unable to work normally and transmit large harmonic currents to the power grid.
For the 48-pulse rectification, the currently existing method is to form a 48-pulse rectification through phase shifting and parallel connection of four sets of 12-pulse rectifying devices. Four sets of 12-pulse rectifying devices include four rectifier transformers with two valve-side output windings. The floor area of rectifier transformers and the integral engineering cost can be reduced by reducing the number of rectifier transformers, i.e. increasing the set number of valve-side output windings of a single rectifier transformer and ensuring the same rectification effect.